1. Field of the Invention
The present invention relates to an ultrasonic oscillator, and more particularly to an ultrasonic oscillator for generating ultrasonic oscillations by an electricity-to-mechanical energy conversion element thereof such as a piezoelectric element or an electrostrictive element.
2. Related Art Statement
Hitherto, a variety of ultrasonic oscillators have been suggested. Examples of the ultrasonic oscillators disclosed in Japanese Patent Laid-Open No. 2-36776 are shown in FIGS. 116 and 117.
The ultrasonic oscillator according to the foregoing disclosure is a piezoelectric elliptic motion oscillator having an arrangement that a penetration member 106 is used to penetrate the following members to secure them: a first cylindrical piezoelectric oscillator 101, a second cylindrical piezoelectric oscillator 102, a metal cylinder 103 disposed between the two inner terminative ends of the respective cylindrical piezoelectric oscillators 101 and 102, a second metal cylinder 104 and a third metal cylinder 105 respectively disposed at the outer terminative ends of the piezoelectric oscillators 101 and 102.
The first cylindrical piezoelectric oscillator 101 is polarized at a first boundary plane 107 which bisections the first cylindrical piezoelectric oscillator 101 in a direction along the central axis of the first cylindrical piezoelectric oscillator 101 and a second boundary plane 108 which bisections the same in a direction of the thickness of the first cylindrical piezoelectric oscillator 101, the polarization being so made that the divided sections have opposite polarities in the direction of the thickness.
The second cylindrical piezoelectric oscillator 102 is polarized by a third boundary plane 109 which bisections the second cylindrical piezoelectric oscillator 102 in a direction along its central axis and which makes a certain angle from the first boundary plane 107 and a fourth boundary plane 110 which bisections the same in a direction of the thickness of the second cylindrical piezoelectric oscillator 102. The polarization is so made that the divided sections have opposite polarities in the direction of the thickness. Furthermore, first and second intermediate terminal plates 111 are respectively disposed on the second and the fourth boundary planes 108 and 110. By applying AC voltages having different phases to the intermediate terminal plates 111 while making the first, second and third metal cylinders 103, 104 and 105 to be common earth, elliptic, including circular, motions can be actuated at the two terminative ends of the aforesaid piezoelectric elliptic motion oscillator.
It should be noted that the aforesaid conventional ultrasonic oscillator rotates relative to the central axis thereof while generating linear bending motions as shown in FIGS. 118 to 120. An ultrasonic oscillator of the aforesaid type has the cylindrical piezoelectric oscillators disposed at the nodes of the oscillations and closely secured by a penetration member such as a bolt. Although a large mechanical quality coefficient (hereinafter abbreviated to "mechanical Qm") showing sharpness of the mechanical oscillation in the vicinity of the resonant frequency can be attained, the elliptic motion oscillator easily generates heat because a large electric current flows therein immediately after the operation has been commenced.
The heat generation raises the impedance of the resonant frequency, causing a contrary phenomenon to take place in that the electric current cannot easily flow. As a result, the oscillation amplitude of the oscillator is reduced. Furthermore, the rigidity of the penetration member, which is also oscillated due to the bending oscillations, restricts the bending oscillations, causing the oscillation amplitude to be reduced. As described above, the penetration member used in the conventional structure is able to raise the mechanical Qm of the oscillator but it restricts the heat generation and reduces the oscillation amplitude.
Moreover, if enlargement of the amplitude realized at the time of the resonance is desired, the ultrasonic oscillator disclosed in Japanese Patent Laid-Open No. 2-36776 and constituted as described above encounters a problem in that the size of the oscillator is inevitably enlarged because the amplitude enlargement can be realized by only lengthening the overall length as well as raising the voltage to be applied to the piezoelectric oscillator.
Furthermore, the fact that the bending oscillations are restricted by the rigidity of the bolt serving as the penetration member and that the resonator and the piezoelectric element are tightened and fixed by the bolt and nuts having large contact areas will cause the aforesaid conventional ultrasonic oscillator to have another problem that the oscillation amplitude at the outer peripheral of the end surface is undesirably restricted by the fixing force.
In addition, the boundaries between the piezoelectric elements of the conventional ultrasonic oscillator must be accurately aligned to one another. For example, if the first boundary plane 107 of the first cylindrical piezoelectric oscillator 101 and the third boundary plane 109 of the second cylindrical piezoelectric oscillator 102 shown in FIG. 116 are deviated from each other as designated by reference numerals 101a and 101Fa of FIG. 121, the resonant frequency of the first piezoelectric element group and that of the second piezoelectric element group do not coincide with each other. As a result, a spurious mode (another resonance mode generated in the main resonance mode to be employed) is undesirably generated, causing the mechanical quality coefficient (Qm) of the oscillator to be decreased. Hence, the efficiency of an ultrasonic motor deteriorates and desired strong torque cannot be obtained.
Furthermore, the cost cannot be reduced if the angular difference between the boundary lines of the first piezoelectric element group and the second piezoelectric element group cannot be aligned to each other because the phase difference of the two-phase sine wave voltage of the drive circuit must be varied and adjusted.
It should be noted that the aforesaid conventional ultrasonic oscillator generates the primary mode bending oscillation as shown in FIGS. 118 to 120 and therefore it rotates relative to the central axis thereof.
FIG. 123 is a schematic view of the aforesaid oscillation mode. Hitherto, an ultrasonic oscillator of the aforesaid type has an arrangement that the two ends thereof are used as free ends and two nodes of the oscillation appear at positions 0.224 L from the two ends of the ultrasonic oscillator (L is the overall length of the ultrasonic oscillator), causing the wavelength of the oscillations to be 1.104 L. However, a small size ultrasonic motor of a type having a short overall length encounters a problem in that the operation frequency (resonant frequency) rises excessively, therefore the operation circuit becomes too complicated and heat can easily be generated. In particular, the small size ultrasonic oscillator encounters a problem in that the impedance at the resonant point thereof is raised if the heat is generated and the mechanical Qm is reduced excessively. As a result, the torque will be weakened and the efficiency deteriorates.
The ultrasonic motor has been utilized recently in place of a so-called electromagnetic motor which generates mechanical rotational force by utilizing electromagnetic force. The ultrasonic motor has overcome a problem experienced with the electromagnetic motor in that it rotates at a high speed while generating weak torque and therefore a large quantity of energy is lost in a reducing mechanism thereof. The ultrasonic motor has an electricity-to-mechanical energy conversion element such as a piezoelectric element.
As disclosed in, for example, Japanese Patent Laid-Open No. 2-311184, the ultrasonic motor has an arrangement that an ultrasonic oscillator (stator) is constituted by fixing an elongated plate-like piezoelectric element to the side surface of a cylindrical elastic member, and a rotor serving as a member to be driven, which is pressed by a pressing means, is disposed on the end surface of the ultrasonic oscillator.
When two alternating voltages, the phases of which delay by 90.degree., are applied to the piezoelectric element of the ultrasonic motor constituted as described above, bending oscillations having two nodes are generated, the bending oscillations being then converted into rotational motion around the axis of the aforesaid elastic member. Then, a point of an end surface of the oscillator comes into point-contact with the rotor, causing the rotor to be rotated. Incidentally, the aforesaid conventional ultrasonic motor has an arrangement that the positions of the two nodes of the oscillations are supported and secured to the case of the motor.
However, the conventional ultrasonic motor of a type for rotating the member to be driven by utilizing the rotation of the bending oscillation has an arrangement that the top surface of an oscillator 141 and a member 142 to be driven come in contact with each other at a point 143 as shown in the contact state shown in FIG. 124. Hence, there arises a problem in that the characteristics (the torque and the rotational speed and the like) are easily affected by a state of the contact point. What is worse, the sole contact point is positioned at the periphery of the end surface of the oscillator (that is, the edge portion), causing the machining accuracy of the edge portion, and more particularly the flatness to affect the performance of the motor. However, it is generally very difficult to improve the machining accuracy.
Since vertical pressure from the pressing means and horizontal frictional force acting due to the load are applied to the sole contact point, the force per unit area at the contact point is enlarged, causing a problem to take place in that the aforesaid edge portion can easily be worn.
On the other hand, an ultrasonic motor disclosed in Japanese Patent Laid-Open No. 4-91670 has a Langevin ultrasonic oscillator to serve as the oscillation generating means. The Langevin ultrasonic oscillator has an oscillator comprising an annular piezoelectric element serving as an electricity-to-thermal energy conversion element and electrodes for applying voltage to the piezoelectric element. The Langevin ultrasonic oscillator is constituted in such a manner that the piezoelectric element is disposed and bonded between the electrode plates. The electrode plate has a voltage application portion constituted by forming a conductive metal thin plate into substantially the same shape as that of the piezoelectric element, the electrode plate having a terminal portion for connecting a lead wire at a terminative end thereof.
Since the voltage application means which uses the conductive metal thin plate can easily be manufactured and satisfactory contact with the piezoelectric element can be established, it has been used widely.
However, the aforesaid conventional ultrasonic motor encounters a problem in that its terminal portion having a small volume inevitably has a large space because the terminal portion of the electrode plate projects over outer periphery of the oscillator. Moreover, in the case where the piezoelectric elements are stacked to form a multilayer, a multiplicity of terminal portions project over the outer periphery of the ultrasonic oscillator, causing a necessity to arise in that a large number of lead wires must be connected to the terminal portions. Hence, a further critical problem of the large space to take place and the wiring process, of course, becomes too complicated. The aforesaid facts are critical problems particularly for small size ultrasonic motors (ultrasonic motors having a diameter of 10 mm or less).
Another technical means has been disclosed in Japanese Patent Laid-Open No. 4-91671, the technical means having an arrangement that a fastening means for fastening each component member of the ultrasonic oscillator comprises a supporting member disposed at one or more terminative ends thereof so as to support the body of the oscillator.
The technical means has an arrangement that the frequency in the specific mode under the boundary conditions of the supporting member is made to be different from the frequency for operating the ultrasonic oscillator. As a result of use of the supporting member thus arranged, a friction loss is prevented because the supporting member is, due to the elastic deformation thereof, enabled to follow the displacement of the oscillator due to oscillations, causing an effect to be obtained in that the efficiency of the motor is improved.
Although the aforesaid conventional supporting method is significantly effective to prevent the friction loss taking place between the supporting member and the fixing member, a problem takes place in that the oscillator cannot be supported stably because the supporting members have elasticity. Another problem takes place in that the supporting member can be broken because stress of bending type ultrasonic oscillations concentrically acts on the supporting member.
Then, the ultrasonic motor disclosed in Japanese Patent Laid-Open No. 4-91672 will now be described.
The ultrasonic motor disclosed as described above has a Langevin ultrasonic oscillator comprising first and second piezoelectric elements arranged to be applied with sine wave voltages having phases delayed by 90.degree. from each other so that bending oscillations are generated around the central axis thereof. The bending oscillations thus generated rotate a movable member disposed on an end surface of the ultrasonic oscillator and serving as a follower member.
The movable member has a groove formed therein in order to efficiently receive the oscillations transmitted from the ultrasonic oscillator. As a result, the state of contact between the ultrasonic oscillator and the movable member is made to be uniform, causing an effect to be obtained in that an undesirable energy loss can be prevented. Therefore, the efficiency of the ultrasonic motor can be improved.
However, the conventional ultrasonic motor thus arranged encounters a problem in that the oscillating energy leaks outwards because the ultrasonic oscillations for operating the movable member undesirably pass through the movable member and the ultrasonic oscillations are transmitted to a supporting shaft or an output receiving means or a means for pressing the movable member.
The outward leak of oscillating energy causes problems to take place in that the output obtainable from the movable member can be reduced, the other component member can be undesirably oscillated and screws for use in fixing the components are loosened.